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Membrane surface-charge titration probed by gramicidin A channel conductance
T K Rostovtseva1, V M Aguilella, I Vodyanoy
1Laboratory of Physical and Structural Biology, NICHD, National Institutes of Health, Bethesda, Maryland 20892-5626 USA.
Biophysical Journal
|September 24, 1998
Summary
We investigated how lipid bilayer surface charge affects gramicidin A channel conductance. Our findings reveal that while charge influences conductance, other factors like ion saturation and lipid structure also play significant roles.
Area of Science:
- Biophysics
- Membrane Biophysics
- Ion Channel Function
Background:
- Lipid bilayer surface charge significantly impacts membrane protein function.
- Gramicidin A is a well-characterized channel forming peptide used to study ion transport.
- Understanding electrostatic interactions at the membrane interface is crucial for cell physiology.
Purpose of the Study:
- To quantify the influence of lipid bilayer surface charge on gramicidin A channel conductance.
- To develop a method for determining the intrinsic pKa of charged lipids.
- To differentiate electrostatic effects from other factors affecting channel function.
Main Methods:
- Manipulating lipid bilayer surface charge via pH titration and dilution with neutral lipids.
- Employing diphytanoyl phosphatidylserine (PS) and diphytanoyl phosphatidylcholine (PC) lipid bilayers.
- Utilizing smeared charge models and Poisson-Boltzmann equations to analyze ion distribution.
Main Results:
- Lipid charge titration effects on conductance were masked by Cs+ ion saturation and proton concentration at low pH.
- A novel method for "bilayer pKa" determination yielded an intrinsic pKa for PS lipids between 2.5-3.0.
- Diluting charged lipids resulted in a greater conductance decrease than predicted electrostatically, indicating additional structural influences.
Conclusions:
- Gramicidin A channel conductance is modulated by lipid surface charge, but electrostatic models alone are insufficient.
- Intrinsic lipid pKa values can be determined by separating charge contributions.
- Lipid-dependent structural factors, beyond electrostatics, are critical determinants of gramicidin A channel conductance.